WASHINGTON—It was one of the greatest calamities of all time: Something turned up the Earth's thermostat, touching off a monstrous heat wave that killed many animals and drove others far from their homes to seek cooler climes.

This catastrophe occurred 55 million years ago, after the age of the dinosaurs and long before humans appeared. But scientists warn that today's global warming means that it could be happening again.

The ancient hot spell, which lasted 50,000 to 100,000 years, goes by the unwieldy name of Paleocene-Eocene Thermal Maximum. It was caused by a sudden—in geological terms—doubling or tripling of carbon dioxide and other greenhouse gases in the atmosphere. Climate scientists say the result was a massive increase of 10 to 12 degrees Fahrenheit—even higher near the poles—above the prevailing temperature.

"In certain regards, the PETM is very similar to what is happening right now," said Gerald Dickens, an earth scientist at Rice University in Houston. "Just like now, a huge amount of carbon rapidly entered the ocean or atmosphere. The most notable difference is the rate. Things are happening much faster now than during the PETM."

Most scientists attribute much of today's global warming to the burning of carbon-rich fossil fuels in factories, cars and trucks. If the present trend continues, Dickens said, the world will add as much carbon to the atmosphere in 500 years—from 1800 to 2300—as the PETM did over 10,000 years.

The long-ago heat wave "shows without a doubt that if you pump a bunch of greenhouse gases into the atmosphere, the planet warms," Matthew Huber, an earth scientist at Purdue University in West Lafayette, Ind., wrote in the June 1 edition of the journal Nature.

Scientists realized the speed and extent of the prehistoric carbon explosion only recently.

A blizzard of scientific papers "reflects the community's excitement at discovering an extraordinary perturbation in biogeochemical systems that was unimaginable 10 years ago," James Zachos, an earth scientist at the University of California, Santa Cruz, declared on his Web site.

"The evidence for dramatic warming during the event is overwhelming," Dickens said. "It is witnessed in all the oceans and continents."

For example:

_Digging in Wyoming's Bighorn Basin, Scott Wing, a paleontologist at the Smithsonian's National Museum of Natural History in Washington, found fossilized leaves from ancient bean plants that he said had migrated 1,000 miles north from the latitude of Louisiana to escape the heat.

_Zachos led a team of scientists that drilled into the South Atlantic ocean floor and found thick stripes of red clay that had lost their carbon to the atmosphere at the time of the PETM.

_Fossils of tropical algae buried under the Arctic Ocean showed that waters there reached a balmy 75 degrees, according to Huber.

"Subtropical conditions arrived in the Arctic during this time," Huber reported in Nature.

Many new species of mammals arose during the PETM and spread to new areas of the world, altering the course of evolution.

But the unusual warmth also caused the loss of many deep-sea species. "It was the most severe extinction in the last 90 million years," according to Gabriel Bowen, another Purdue geologist.

"This state may in some ways represent our global environmental future, and it is therefore critical that we begin to characterize it," Bowen wrote in a paper that the American Geophysical Union published in April.

Scientists are still debating the cause of the 55-million-year-old greenhouse explosion. At least seven possible triggers have been reported, but none of them alone seems enough to explain the event. Wing said it may have required several of the triggers to force such huge changes in the ocean and the atmosphere.

Andrew Kurtz, an earth scientist at Boston University, suggested that falling sea levels may have exposed vast plains of peat—an early form of coal—which caught fire and burned for thousands of years, releasing billions of tons of stored carbon.

Another idea, by Henrik Svensen, a Norwegian geologist, is that volcanoes under the North Atlantic belched enough carbon-rich lava to alter the atmosphere.

Dickens suggested that frozen methane, a powerful greenhouse gas made of carbon and hydrogen, escaped from below the seafloor and caused the warming.

The PETM "isn't a crystal ball that tells us what's going to happen," Wing said. "But at least it gives us a clue as to what might happen."

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